The present invention relates to a color liquid crystal display capable of being driven by an interlace drive system.
Liquid crystal displays are utilized for various purposes for their compactness and ability to function at low power consumption. Recently, various color liquid crystal displays capable of displaying color pictures in a high picture quality have been proposed.
Liquid crystal displays are driven by the interlace drive system or the noninterlace drive system. The noninterlace drive system scans sequentially rows of pixels arranged in a matrix one at a time. The interlace drive system scans every other row of pixels, i.e., odd rows of pixels in a field first, and then even rows of pixels of the same field.
The frequency of driving of pixels in unit time by the interlace drive system is half that of driving of pixels in unit time by the noninterlace drive system, and hence the interlace drive system enables the liquid crystal display at a low power consumption. However, if the pixels of an NTSC TV monitor employing a liquid crystal display panel are driven by the interlace drive system, the so-called line crawling, i.e., visual disturbance on a TV screen in which line crawling occurs on the TV screen, is liable to occur to deteriorate picture quality. The followings are possible causes of line crawling on a liquid crystal display panel driven by the interlace drive system.
(1) Usually, liquid crystal pixels are driven by a.c. power. Referring to FIGS. 5A to 5D illustrating the interlace drive system which drives pixels by a.c. power, pixels on every other row are scanned in each field; i.e., pixels on odd rows are scanned in odd fields, and pixels on even rows are scanned in even fields. Rows to be driven are changed between the fields. In FIGS. 5A to 5D, R, G and B indicate red, green, and blue pixels, respectively, a square (.quadrature.) indicates a pixel to which a positive driving voltage is applied, and a triangle (.DELTA.) indicates a pixel to which a negative voltage is applied. It is noted from the observation of pixels of the same color on each row that pixels driven by a positive driving voltage and those driven by a negative voltage are arranged alternately. In FIGS. 5A to 5D, each of oblique lines passes adjacent green pixels G in each field driven by a positive driving voltage. PA1 (2) The amplitude of a flicker which occurs when the liquid crystal display is driven by the interlace drive system is greater than that of a flicker which occurs when the liquid crystal display is driven by the noninterlace drive system because of the following reasons.
Each pixel has an optical characteristic expressed by an even function of the driving voltage; that is, the optical characteristic of each pixel is dependent on the absolute value of the amplitude of the driving voltage applied to the pixel. Therefore, the absolute value of the positive driving voltage to be applied to the pixel and that of the negative driving voltage to be applied to the same pixel must be equal to each other to keep constant the brightness of the pixel.
However, a d.c. offset voltage is applied inevitably to the pixel in addition to an a.c. driving voltage as shown in FIG. 7 due to the characteristics of the device when the liquid crystal display employs TFTs (thin-film transistors) as shown in FIG. 6. Consequently, the transmittance of each pixel varies in a frequency half the pixel driving frequency, so that the brightness of the pixel varies and the pixel flickers. The pixel driving frequency of the noninterlace drive system is 60 Hz and the pixel driving frequency of the interlace drive system is 30 Hz. The flickering frequency therefore is 30 Hz for the noninterlace drive system and 15 Hz for the interlace drive system. Human visual sensitivity with a flicker increases with the decrease of the frequency of the flicker. Therefore, flickers that occur when the liquid crystal display is driven by the interlace drive system is more visually recognizable than flickers that occur when the liquid crystal display is driven by the noninterlace drive system.
If the variation of the transmittance with time is easily visually recognizable, the variation of the same in space also is easily visually recognizable, so that line crawling occurs. For example, the pixels G on oblique lines in the first field shown in FIG. 5A are driven by a positive driving voltage, and the same pixels G in the third field shown in FIG. 5C are driven by a negative driving voltage. Therefore, if a d.c. offset is superposed on the a.c. driving voltage and the absolute value of the positive driving voltage is greater than that of the negative driving voltage, the pixels on the oblique lines are bright in the first field shown in FIG. 5A and are darker in the third field than those in the first field. If flicker is viewed in a space, the oblique lines shown in FIGS. 5A to 5D flicker, causing line crawling.
When a liquid crystal display as shown in FIG. 6 is driven by the noninterlace drive system, a gate voltage of 16.7 ms in period as shown in FIG. 8A is applied to the TFT1 of each pixel. The voltage holding time of the pixel, therefore, is 16.7 ms when the liquid crystal display is driven by the noninterlace drive system. When the liquid crystal display is driven by the interlace drive system, a gate voltage is applied to the TFT of each pixel at a period of 33.3 ms as shown in FIG. 8B and hence the voltage holding time of each pixel is 33.3 ms.
A leakage current flows through a leakage resistor R even while the TFT employed in the liquid crystal display is in an OFF state. The quantity of charges discharged by a liquid crystal capacitor C.sub.LC when the liquid crystal display is driven by the interlace drive system is greater than that when the liquid crystal display is driven by the noninterlace drive system. Therefore, as shown in FIGS. 8A and 8B, the amplitude of the flicker when the liquid crystal display is driven by the interlace drive system is greater than that when the liquid crystal display is driven by the noninterlace drive system.
As is apparent from the foregoing description, the frequency and the amplitude of the flicker in a liquid crystal display driven by the interlace drive system, as compared with those of the flicker in a liquid crystal display driven by the noninterlace drive system, is disadvantageous in respect of visual characteristics. Therefore, the interlace drive system employs an inverting drive system, such as a pixel inverting drive system, to deal with flickers of the same phase on the screen. However, the problem relating to deterioration of the picture quality by line crawling has not been solved yet.